1. Field of the Invention
The present invention relates to a nitride-based light-emitting device and a method of manufacturing the same, and more particularly, it relates to a nitride-based light-emitting device comprising a nitride-based semiconductor element layer and a method of manufacturing the same.
2. Description of the Background Art
A nitride-based light-emitting device such as a nitride-based light-emitting diode device comprising a nitride-based semiconductor element layer consisting of a nitride-based semiconductor is actively developed at present. In order to employ a nitride-based light-emitting diode device as the light source for a lighting fixture, improvement of the light output characteristic of the nitride-based light-emitting diode device and increase of an applied current have recently been developed in particular. In order to form such a nitride-based light-emitting diode device, a nitride-based semiconductor element layer is grown on a sapphire substrate requiring a more reasonable cost than a high-priced nitride-based semiconductor substrate of GaN or the like.
FIG. 35 is a sectional view showing the structure of a conventional nitride-based light-emitting diode device. Referring to FIG. 35, a buffer layer 102, an underlayer 103, an n-type contact layer 104, an n-type cladding layer 105 and an active layer 106 are successively formed on an insulating sapphire substrate 101 in the conventional nitride-based light-emitting diode device. A cap layer 107, a p-type cladding layer 108 and a p-type contact layer 109 are successively formed on the active layer 106. The n-type contact layer 104, the n-type cladding layer 105, the active layer 106, the cap layer 107, the p-type cladding layer 108 and the p-type contact layer 109 constitute a nitride-based semiconductor element layer 100.
A prescribed region of the nitride-based semiconductor element layer 100 is removed between the upper surface of the p-type contact layer 109 and a portion of an intermediate depth of the n-type contact layer 104, for partially exposing the n-type contact layer 104. A p-side electrode 110 is formed on the p-type contact layer 109, while an n-side electrode 111 is formed on a prescribed region of the exposed surface of the n-type contact layer 104.
In the conventional nitride-based light-emitting diode device shown in FIG. 35, as hereinabove described, the p-side electrode 110 and the n-side electrode 11 are taken out from the surface of the nitride-based semiconductor element layer 100 opposite to the sapphire substrate 101. In order to increase the emission area for improving the light output characteristic, therefore, light must be emitted from the side of the sapphire substrate 101 formed with neither the p-side electrode 110 nor the n-side electrode 111. Therefore, flip chip bonding is employed for mounting the conventional nitride-based light-emitting diode device shown in FIG. 35 to a base (not shown) from the side of the p-side electrode 110 and the n-side electrode 111.
However, the conventional nitride-based light-emitting diode device shown in FIG. 35 has a step between the p-side electrode 110 formed on the p-type contact layer 109 and the n-side electrode 111 formed on the exposed surface of the n-type contact layer 104. In order to assemble the conventional nitride-based light-emitting diode device shown in FIG. 35 by flip chip bonding, therefore, it is necessary to provide the base with a step portion corresponding to the step between the p-side electrode 110 and the n-side electrode 111 with precise position control for attaining coincidence between the positions of the step portion and the p-side electrode 110 and the n-side electrode 111. Therefore, the manufacturing yield is disadvantageously reduced. Further, the thermal conductivity of the sapphire substrate 101 of the conventional nitride-based light-emitting diode device shown in FIG. 35 is so low that it is disadvantageously difficult to sufficiently radiate heat generated in the nitride-based semiconductor element layer 100.
Therefore, generally proposed is a nitride-based light-emitting diode device formed by growing a nitride-based semiconductor element layer on a sapphire substrate and thereafter bonding a cleavable conductive substrate of GaAs or the like to the nitride-based semiconductor element layer and removing the sapphire substrate, as disclosed in Japanese Patent Laying-Open No. 9-8403 (1997), for example. In the aforementioned nitride-based light-emitting diode device disclosed in Japanese Patent Laying-Open No. 9-8403, a p-side electrode and an n-side electrode can be oppositely arranged through the nitride-based semiconductor element layer due to the employment of the conductive substrate. Therefore, the nitride-based light-emitting diode device may not be assembled by flip chip bonding requiring precise position control dissimilarly to the case of employing the insulating sapphire substrate, whereby the manufacturing yield can be improved.
In the aforementioned nitride-based light-emitting diode device disclosed in Japanese Patent Laying-Open No. 9-8403, however, the thermal conductivity of the conductive substrate of GaAs or the like is so insufficient that it is disadvantageously difficult to sufficiently radiate heat generated in the nitride-based semiconductor element layer, similarly to the case of employing the sapphire substrate. Consequently, the light output characteristic is disadvantageously reduced by heat when a high current is fed to the nitride-based light-emitting diode device. Further, difference between the linear expansion coefficients of the conductive substrate of GaAs or the like and the nitride-based semiconductor element layer of GaN or the like is so large that the nitride-based light-emitting diode device is warped in the manufacturing process due to the difference between the linear expansion coefficients. Consequently, the manufacturing yield is disadvantageously reduced due to the warping of the diode device.